FIG. 1 schematically illustrates such a pressurized water nuclear reactor 1, which comprises in a conventional manner:                a core 2 divided into an upper area and a lower area and producing power,        steam generators 3, a single generator being represented,        a turbine 4 coupled to an electric power generator 5, and        a condenser 6.        
The reactor 1 also comprises a primary circuit 8 equipped with pumps 9, a single pump being represented, and in which pressurized water circulates, along the path indicated by the arrows. This water rises particularly to the core 2 to be heated therein while assuring the cooling of the core 2. The water also assures a function of moderation, in other words of slowing down the neutrons produced by the nuclear fuel.
The primary circuit 8 further comprises a pressuriser 10 making it possible to regulate the pressure of the water circulating in the primary circuit 8.
The water of the primary circuit 8 also supplies the steam generators 3 where it is cooled while assuring the vaporisation of water circulating in a secondary circuit 12.
The steam produced by the generators 3 is channelled by the secondary circuit 12 to the turbine 4 then to the condenser 6 where said steam is condensed by indirect heat exchange with the cooling water circulating in the condenser 6.
The secondary circuit 12 comprises, downstream of the condenser 6, a pump 13 and a heater 14.
The core 2 comprises fuel assemblies 16 which are loaded in a vessel 18. A single assembly 16 is represented in FIG. 1, but the core comprises a plurality of assemblies 16.
The fuel assemblies 16 comprise nuclear fuel rods formed, in a conventional manner, of an alloy cladding, based on zirconium, enclosing a stack of nuclear fuel pellets based on uranium oxide or a mixture of uranium oxide and plutonium oxide.
The reactor 2 comprises control rods 20, also known as control rod clusters, for controlling the reactivity of the core, which are arranged in the vessel 18, above certain assemblies 16, and which are capable of occupying a plurality of insertion positions in the core. A single rod 20 is represented in FIG. 1, but the core 2 comprises several tens of control rod clusters 20. The control rods 20 can be moved vertically by mechanisms 22 so as to be inserted, in different insertion positions, in the fuel assemblies 16 that they overhang.
In a conventional manner, each control rod 20 comprises a plurality of control pencils made of neutron absorbing material.
Thus, the vertical movement, or insertion state, of each rod 20 inside the fuel assemblies 16 makes it possible to regulate the reactivity of the core of the reactor 1, thereby authorising variations in the overall power supplied by the core 2, from zero power up to rated power (hereafter noted RP).
It may prove to be useful, in fact, particularly in countries such as France where 80% of the electricity is produced by nuclear reactors, that the overall power supplied by the reactors varies in order to adapt to the needs of the grid that they supply; this is then known as grid monitoring or load follow.
During load follow, the power produced by the reactor is regulated so as to correspond to a programme pre-established by the service operating the grid.
The adjustment of the power supplied by the reactor is achieved by operating means positioning control rods constituted of neutrophage element in different insertion positions in the core so as to absorb more or less the neutrons and/or by optionally adjusting the concentration of a neutron absorbing compound, such as boron, in the primary coolant, as a function of the desired power and/or measurements from the instrumentation of the core of the reactor.
For example, the operating means are formed of a set of electronic and electrical equipment which, from measurements from instrumentation chains and by comparing them to limit levels, elaborate orders of movement of control rods 20 and/or modification of the boron concentration in the primary coolant by injection of water (dilution) or boron (boronation).
Different modes of operating a pressurized water nuclear reactor are known. Generally speaking, the operation consists in controlling and regulating to the minimum the average temperature of the primary coolant Tav and the distribution of power (thermal and neutronic) and in particular the axial distribution of power DA in order to avoid the formation of a power imbalance between the upper area and the lower area of the core.
The methods of regulation of these parameters vary as a function of the different operating modes used. Generally speaking, the average temperature Tav is regulated by the movement of the control rods 20 as a function of different parameters such as the power required at the turbine, the standard value of the temperature of the coolant, and/or optionally by modification of the boron concentration in the primary coolant, which makes it possible indirectly to adapt the positions of the control rods 20 to a desired position, particularly in order to obtain an axial distribution of desired power DA and/or a capacity of rapid rise in the power of the core to the desired power.
The choice of the mode for operating a nuclear reactor is determined by taking into consideration the fact that the action of the control rods has immediate effects, whereas the action by injection of boron in solution is comparatively slower.
Moreover, the increase in the boron concentration in solution in the primary coolant requires boric acid storage and injection means and thus imposes additional design constraints.
Thus, there is a tendency only to use the injection of boron or water in solution to correct the long term effects on the operating reactivity of the reactor, in other words essentially the xenon effect and the ageing of the fuel.
In order to meet the needs of the grid, the operation of the reactor is thus preferentially carried out by the movement of the control rods.
However, the insertion of the control rods affects, in a prejudicial manner, the axial distribution of power produced in the reactor. This may result in the formation of power peaks in the core as well as the development of oscillations of the xenon concentration in the longer term, favourable to the accentuation of these power peaks, factors intervening in a restrictive manner in the operating procedure and imposing a corrective recourse by modifying the boron concentration in the primary coolant.
Yet, in load follow, in other words with a power production level following a daily curve, and even in slave mode, by remote control, the variations in power production multiply the control actions, with the aforementioned unfavourable consequences, engaging in an important manner the control rod mechanisms and leading to considerable volumes of effluents due to repeated operations of dilution and boronation of the coolant.
In order to meet these difficulties, methods for operating a pressurized water reactor have been developed determining the positions of the control rods in the core, making it possible to limit perturbations of the axial power distribution and resorting to the use of boron, the concentration of which is adjusted so as to mainly compensate the effects of the release of xenon and the ageing of the fuel rods.
However, this operating method is not always optimised and does not always make it possible to minimise the volumes of effluents as well as the movement of the control rod clusters. In addition, the minimisation of the volumes of effluents as well as the engaging of the control rod insertion mechanisms remains a permanent concern of the operator.